Solar Collection Apparatus and Methods Using Accelerometers and Magnetics Sensors

ABSTRACT

A mirror or other reflecting surface is used for collecting and reflecting incident solar radiation. The mirror is supported for independent motion about a pair of axes. An accelerometer generates signals representative of an amount and direction of motion of the mirror about each of the axes. Motors or other drive mechanisms independently drive the mirror about each of the axes. A tracking device provides information about the current position of the Sun. A control is connected to the accelerometer, the motors and the tracking device for maintaining a predetermined optimum orientation of the mirror as the Sun moves across the sky. Position sensors that sense the position of the mirror relative to the earth&#39;s magnetic field may also be employed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of U.S. application Ser. No.11/763,267 of Mark S. Olsson, filed Jun. 14, 2007.

This application also claims benefit under 35 USC Sections 119(e) and120 to the filing date of U.S. Provisional Application Ser. No.60/807,456 filed by Mark S. Olsson on Jul. 14, 2006.

FIELD OF THE INVENTION

The present invention relates to systems and methods for utilizing theenergy of the Sun, and more particularly, to systems and methods fortracking the Sun to re-direct and concentrate incident solar radiationfor lighting, heating and photovoltaic applications.

BACKGROUND OF THE INVENTION

Increased usage of renewable energy sources such as solar radiation isimportant in reducing dependence upon foreign sources of oil anddecreasing green house gases. Devices have been developed in the pastthat track the motion of the Sun to re-direct and concentrate incidentsolar radiation. FIG. 1 illustrates one example of a prior art devicethat utilizes a parabolic dish mirror 10 with a central axis 12 that ispointed generally toward the Sun 14. Incident solar radiation 22 isreceived and reflected by the parabolic dish mirror 10 and concentratedat its focus 16, where a thermal target (not illustrated) can be mountedso that it can be heated. The parabolic dish mirror 10 is supported forindependent movement by a two-axis tracking support 18 mounted atop asupporting structure 20 such as a tower. Optical encoders (notillustrated) associated with the tracking support 18 provide signalsindicative of the direction and amount of rotation of the parabolic dishmirror 10 so that motor drives and a control system (not illustrated)can be used to track the Sun and increase the efficiency of the energytransfer.

FIG. 2 illustrates another example of a prior art device similar to thedevice of FIG. 1 except that the device of FIG. 2 utilizes a parabolictrough mirror 30. Dashed line 32 illustrates a common plane of the focalline 36 of the parabolic trough mirror 30 and the Sun 14. A single axistracking support 38 carries the parabolic trough mirror 30 and ismounted atop a tower 40. Incident light rays from the Sun such as 42 arecollected and reflected by the parabolic trough mirror 30 andconcentrated on a pipe (not illustrated) that extends along the focalline 36. This allows a heat transfer fluid such as water or liquidsodium to be heated. The heating efficiency can be improved bymechanisms (not illustrated) that cause the parabolic trough mirror 30to pivot and track the Sun. FIG. 3 illustrates another prior art devicethat utilizes a heliostat flat mirror 50 that receives incident lightrays 52 from the Sun 14 and reflects them against a thermal target 58atop a tower 59. Another tower 54 carries a two-axis tracking support 56which supports a flat mirror 50. Drive and control mechanisms (notillustrated) allow the flat mirror 50 to be independently moved about arotate axis 60 (azimuth) and about a tilt axis 62 (elevation) to ensurethat the Sun's rays are reflected onto the target 58 as the Sun movesacross the sky. There are many variations of the foregoing devices, butto date, none has been widely adopted due to the complexity,reliability, accuracy and/or expense of the tracking mechanisms.

SUMMARY OF THE INVENTION

In accordance with the present invention a solar tracking apparatus hasa mirror or other reflecting surface for collecting and reflectingincident solar radiation. The mirror is supported for independent motionabout a pair of axes. An accelerometer generates signals representativeof an amount and direction of motion of the mirror about each of theaxes. Motors or other drive mechanisms independently drive the mirrorabout each of the axes. A tracking device provides information about thecurrent position of the Sun. A control is connected to theaccelerometer, the motors and the tracking device for maintaining apredetermined optimum orientation of the mirror as the Sun moves acrossthe sky.

According to another aspect of the present invention, one or moremagnetic field sensors are used to sense rotation around anapproximately vertical axis and one or more accelerometers are used tosense tilt around an approximately horizontal axis to provide signalsindicative of heliostat mirror position in a coordinate system to acontrol system. The control system allows for precise positioning of aheliostat mirror and the directing of solar energy in a desireddirection.

According to another aspect of the present invention, three orthogonalmagnetic field sensors are used to sense rotation around anapproximately vertical axis and three orthogonal accelerometers are usedto sense tilt around an approximately horizontal axis to provide signalsindicative of heliostat mirror position in a coordinate system to acontrol system. The control system allows for precise positioning of aheliostat mirror and the directing solar energy in a desired direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate examples of prior art solar radiation collectingand redirecting devices.

FIG. 4 illustrates a first embodiment of the present invention thatutilizes a flat mirror to heat a target.

FIG. 5 illustrates a second embodiment of the present invention thatutilizes an array of mirrors to reflect solar radiation through askylight.

FIG. 6 illustrates an alternate embodiment wherein an array of flattracking mirrors reflects incident solar radiation through the windowsof a house to provide light and heat.

FIG. 7 illustrates another embodiment that utilizes an array ofheliostat mirrors to heat a thermal target.

FIG. 8 illustrates another embodiment that utilizes a plurality ofheliostat mirrors to reflect solar radiation onto a high temperaturephotovoltaic panel.

FIG. 9 is a block diagram of another embodiment in which a networkcontroller controls a plurality of mirror nodes.

FIG. 10 illustrates another embodiment in which a heliostat mirror ispositioned to reflect incident solar radiation onto a target via avertical array of photo-sensors.

FIG. 11 is a block diagram illustrating one embodiment of the mirrorcontroller network node of the embodiment of FIG. 4.

FIG. 12 is a flow diagram illustrating one embodiment of a method ofoperation of the control of FIG. 11.

FIG. 13 is a flow diagram of another embodiment of a method of operationof a solar tracking 20 device in accordance with the present invention.

FIG. 14 is a front isometric view of another embodiment that utilizes aweight-tensioned device to pivot the mirror.

FIG. 15 is a back isometric view of the embodiment illustrated in FIG.14.

FIG. 16 is a front elevation view of the embodiment illustrated in FIG.14.

FIG. 17 is a back elevation view of the embodiment illustrated in FIG.14.

FIG. 18 is a side elevation view of the embodiment illustrated in FIG.14.

FIG. 19 is an exploded back isometric view of the embodiment illustratedin FIG. 14.

FIG. 20 is a vertical sectional view (stepped cut) of the embodimentillustrated in FIG. 14 showing internal components thereof.

FIG. 21 illustrates another embodiment that employs magnetic sensors andaccelerometers.

FIG. 22 is a block diagram of the control system of the embodiment ofFIG. 21.

FIG. 23 is a block diagram of control circuitry.

FIG. 24 is a flow chart of data integration of magnetic sensor andaccelerometer data and their resolution.

DETAILED DESCRIPTION

The entire disclosure of U.S. patent application Ser. No. 11/763,267 ofMark S. Olsson, filed Jun. 14, 2007, and published Jan. 17, 2008 asUS-2008-0011288-A1 is hereby incorporated by reference.

FIG. 4 illustrates a first embodiment of the present invention thatutilizes a flat mirror to heat a target. A solar tracking apparatus hasa reflective surface in the form of a mirror 70 for collecting andreflecting incident solar radiation 82 from the Sun 14. The mirror inthis embodiment has a planar configuration, although this embodimentcould be adapted to use other mirror configurations including parabolicdish, parabolic trough, etc. in order to concentrate the incident solarradiation. The mirror could be conventional silver coated glass, orcould be plastic, or could be Mylar® polyester film on a supportsubstrate, or some other form of reflective material that is durable,lightweight and inexpensive.

The mirror 70 (FIG. 4) is supported by a pair of pivot mechanisms 72 forindependent motion about a pair of tilt axes 88 and 90. The pivotmechanisms 72 are mounted atop a support or tower 76. An accelerometer74 generates signals representative of an amount and direction of motionof the mirror about each of the axes 88 and 90. In effect the Earth'sgravity is sensed and used to provide an indication of the currentorientation of the mirror 70. Electric motors 78 (only one of twoillustrated) independently drive the mirror 70 about each of the axesutilizing, for example, a worm gear 80 and a circular rack gear 81. Amirror controller network node 86 includes a tracking device, typicallyan electronic processor, that provides information about the currentposition of the Sun 14. The mirror controller network node 86 alsoincludes a control that is connected to the accelerometer 74, the motors78 and the tracking device for maintaining a predetermined optimumorientation of the mirror as the Sun moves across the sky. Thearchitecture and method of operation of the mirror controller networknode 86 are discussed hereafter in greater detail. Incident solarradiation with an angle of incidence 96 is reflected off the surface ofthe mirror 70 at an angle of reflection 94 so that it strikes a thermaltarget 84 such as a container or conduit of a heat transfer fluid or anarray of photovoltaic cells.

The accelerometer 74 (FIG. 4) is preferably a micro-electro-mechanicalsystems (MEMS) accelerometer device. Utilizing micro-fabricationtechniques a position sensor component and signal conditioning circuitcan be fabricated on a single integrated circuit chip. Such MEMSaccelerometer devices are relatively inexpensive, durable andsufficiently accurate for purposes of manufacturing commercialembodiments of the present invention. Suitable MEMS accelerometerdevices are the KXM52-1040 dual-axis (XY) MEMS accelerometer device andthe KXM52-1 050 tri-axis (XYZ) MEMS accelerometer device, both of whichare commercially available from Kionix, Inc., 36 20 Thronwood Drive,Ithica, N.Y. 14850 USA. See U.S. Pat. Nos. 6,149,190 granted Nov. 21,2000 to Galvin et al. and 6,792,804 granted Sep. 21, 2004 to Adams etal., both of which are assigned to Kionix, Inc., the entire disclosuresof which are hereby incorporated by reference. Also suitable are theADXL321 (two-axis) and ADXL330 (three-axis) MEMS accelerometer devices,both of which are commercially available from Analog Devices, Inc., One25 Technology Way, Norwood, Mass. 02062 USA. See U.S. Pat. Nos.6,837,107 granted Jan. 4, 2005 to Green and 6,845,665 granted Jan. 25,2005 also to Green, both of which are assigned to Analog Devices, Inc.,the entire disclosures of which are hereby incorporated by reference.

While it is possible over certain rotational limits, with appropriatecalibration and alignment to use single axis sensors, it is desirable touse three axis sensors for both magnetic field and gravity. It isanticipated with ongoing reductions in the cost of sensors and greatersystem integration that three axis sensors will be widely available atlow cost. For example, an Aichi Steel Corporation, Electro-MagneticProducts, AMI601 sensor would be suitable for this application. Seehttp://www.aichi-mi.com/3_products/ami%20catalogue%20e.pdf.

The AMI602, which would also serve in this application, is also fromAichi and is a 6-axis motion sensor which incorporates 3-axismagnetometer and 3-axis accelerometer. The controller IC of the AMI602consists of a circuit for sensor elements, an amplifier capable ofcompensating each sensors offset and setting appropriate sensitivityvalues, a temperature sensor, a 12 bitAD converter, an PC serial outputcircuit, a constant voltage circuit for power control and an 8032micro-processor controlling each circuit.

Again referring to FIG. 4, the pivot mechanisms 72 are configured andarranged so that throughout the useful range of tracking tilts, theaccelerometer 74 is not rotated in an unknown fashion about a verticalaxis. If the accelerometer is rotated about a vertical axis, thepointing direction of the mirror 70 becomes ambiguous or indeterminate.The addition of a magnetic sensor in an alternate embodiment avoids thisproblem, as discussed later. Modern compass MEMS devices which combineaccelerometers with magnetic compass sensors are known in the art, andallow the array to be corrected for tilt, and such a device could beused in place of the accelerometer 74.

It will be understood that a wide variation of modifications of theembodiment illustrated in FIG. 4 are possible. For example, theaccelerometer 74 need not be directly mounted to the mirror but could becoupled thereto through a mechanical or optical linkage. The pivotmechanisms 72 could be replaced with an alternate pivot mechanism suchas a ball and socket or flexible joint, instead of those employingindependently movable mechanical pivots. Thus the mirror 70 need notstrictly rotate about two axes, as is the case with the embodiment ofFIG. 4 wherein rotation of the mirror 70 about one axis rotates theother axis. It will be appreciated that it is not necessary that bothaxes of tilt are substantially in the same horizontal plane when themirror 70 is in a normal or horizontal orientation. Other forms of motormeans for driving the mirror 70 can be employed besides the electricmotor 78 and gears 80 and 81, such as hydraulic and pneumatic systems.The mirror 70 need not move in azimuth and elevation, it beingsufficient that it be capable of independent movement about twonon-parallel axes.

FIG. 5 illustrates a second embodiment of the present invention thatutilizes an array 104 of individual mirrors 106 to reflect solarradiation 110 through a skylight 102 on the roof of a building 100 toprovide internal lighting. This greatly increases the amount of solarradiation otherwise directly entering the interior of the buildingthrough the skylight 102 as illustrated by incident light rays 108. Themirrors 106 may each be independently supported and moved as illustratedin FIG. 4 or they may be simultaneously supported and moved by a commontracking system so that reflected light 114 strikes a fixed angle targetmirror 112 and is reflected as downwardly projected light 116. Theskylight 102 may be of the type sold under the SOLATUBE® trademark whichemploys a conduit with a highly reflective surface. Optionally a hotmirror 118 may be inserted in to the reflected light transmission pathto reflect away the infrared component during the summer to avoidunwanted heating of the interior of the building 100.

FIG. 6 illustrates an alternate embodiment wherein an array 144 of flattracking mirrors 142 reflect incident solar radiation 146 as reflectedradiation 148 that passes through the window 140 of a house to providelight and heat. Again the mirrors 142 are supported and moved in thefashion described in connection with FIG. 4.

FIG. 7 illustrates another embodiment that utilizes an array 170 ofheliostat mirrors 168 to heat a thermal target 162. The amount ofincident solar radiation 164 that is redirected as reflected solarradiation 166 is maximized by mounting an accelerometer 160 on eachheliostat mirror 168 and using its signals, along with trackinginformation to tilt each mirror 168 about its two-axis tilting support172.

FIG. 8 illustrates another embodiment that utilizes a plurality ofheliostat mirrors 206 equipped as described in connection with FIG. 4 inorder to re-direct a maximum amount of incident solar radiation 202 asreflected radiation 204 onto a high temperature photovoltaic panel 200.

FIG. 9 is a block diagram of another embodiment in which a networkcontroller 222 controls a plurality of mirror nodes 220. The networkcontroller 222 may be connected to the mirror nodes 220 by a networklink 226 which may be wired or wireless, fiber optic, laser or any otherwell known data communications scheme. One example is the ZIGBEE™ datalink. Bluetooth links or other wireless means could be used as wellaccording to the scale of the application. An optional mirror nodetraining interface 224 is provided that can be used to load the networkcontroller 222 with tracking data from local or remote sources that givethe predicted location of the Sun throughout the day for a givenlatitude, longitude, date and time. This information is used by thecontroller to compare the actual position of the mirrors with theiroptimum positions so that they can be moved to maximize the collectionand/or concentration of solar radiation. Alternatively this informationmay be preprogrammed into the network controller 222 or the mirrorcontroller network node 86 (FIG. 4).

The present invention differs from conventional heliostats that requirea vertical tracking axis. In the present invention, the Sun is trackedin both azimuth and elevation, however, tracking is required in bothaxes as neither component is separately derived.

FIG. 10 illustrates another embodiment in which a heliostat mirror 246is positioned to re-direct incident solar radiation 250 as reflectedsolar radiation 252 to strike a target 248 utilizing mechanisms similarto those described in connection with FIG. 4. A vertical array ofphoto-sensors 240 detect reflected radiation 252 and their signals areused to position the mirror 246 so that the reflected radiation willstrike the target 248. A Sun hood 254 may be used with each photo-sensor240 to prevent it from detecting significant amounts of incident solarradiation 250. The spacing 242 between the photo-sensors 240 can beoptimized relative to the dimension 244 of the mirror 246.

FIG. 11 is a block diagram illustrating one embodiment of mirrorcontroller network node 86 of the embodiment of FIG. 4. A PICmicro-computer based control 300 provides the basic intelligence andcontrol through appropriate input/output interfaces. Positioninformation is received from the accelerometer 302. First and secondaxis motors 304 and 306 are appropriately driven. AC power or some otherpower source 310 such as solar or battery power provides power to thecontrol 300. In order for the mirror to be optimally pointed, it isnecessary for the control 300 to compare the actual position of themirror to the current position of the Sun and make the appropriateadjustments. Data regarding the predicted location of the Sun ispre-programmed into the control 300, in which case a user interface (notillustrated) is necessary for a user to enter the correct latitude,longitude, date and time during initial set up. This interface could bea keypad or a connection to a PC or PDA, for example. Optionally, aGlobal Positioning System (GPS) and time base receiver 312 may beconnected to the control 300 to provide this information. A wired orwireless network link 308 connects the control to a remote location formonitoring or control.

FIG. 12 is a flow diagram illustrating one embodiment of a method ofoperation of the control of FIG. 11. Initially in step 314 the startingparameters are acquired, including latitude and longitude, time, tiltaxis orientation to the North, and the estimated azimuth and elevationof the mirror. Latitude, longitude and time can be obtained via thenetwork. In step 316 the processor calculates the position of the Sun.In step 318, using signals from the accelerometer, and data from a lookup table, the control calculates the movement of the mirror about eachaxis necessary to achieve the optimum orientation. In step 320, themotors are driven by the control the move the mirror as needed to obtainthe optimum orientation. If the accelerometer signals do not indicatemirror motion, an ERROR message is generated and transmitted and/ordisplayed. In step 322, the control continues to track the Sun in orderto engage the target.

FIG. 13 is a flow diagram of another embodiment of a method of operationof a solar tracking device in accordance with the present invention.

FIGS. 14-20 illustrate another embodiment of the present invention thatutilizes weight-tensioned mechanisms to pivot the mirror. The embodiment400 includes a planar square mirror 402 whose corners are supported byfour cable hook corners 404. A small yoke 406 (FIGS. 15 and 19) has asquare surface which is secured to the center of the rear side of themirror 402 by suitable adhesive. Small yoke 406 is connected forindependent rotation about two axes to a tall yoke 408 by a cross piece410. The base of the tall yoke 408 is secured by screws 412 and nuts 414(FIG. 19) to a cylindrical cap plate 416. The cylindrical cap plate 416is mounted on the upper end of a support structure in the form of ahollow vertical support post 418.

A lower tension wire 420 (FIG. 18) has one end connected to theuppermost cable hook corner 404 and its other end connected to thelowermost cable hook corner 404. An upper tension wire 422 (FIGS. 18 and19) has an intermediate segment wrapped around an upper drive pulley 424(FIG. 20) and its ends connected to respective ones of the laterallyspaced cable hook corners 404. The lower tension wire 420 is connectedto a lower counter-weight drive assembly 426 (FIG. 20). The uppertension wire 422 is connected to an upper counter-weight drive assembly428 on which the upper drive pulley 424 is mounted. The lower tensionwire 420 passes through large rectangular apertures 430 (FIG. 18) onopposite sides of the lower portion of the support post 418. The uppertension wire 422 passes through large rectangular apertures 432 formedon opposite sides of the upper portion of the support post 418, andspaced ninety degrees from the apertures 430. The intermediate segmentof the lower tension wire is wrapped around a lower drive pulley 434(FIG. 20) mounted on the lower counter-weight drive assembly 426.

Each of the counter-weight drive assemblies 426 and 428 (FIG. 19) has asimilar construction, and therefore, only one need be described. Thelower counter-weight drive assembly 426 includes a lower micro-motor 436(FIG. 20), a lower rotation restraint mechanism 438, a shaft connector440, and a lower worm gear drive 442. These mechanisms allow the lowertension wire 420 to be driven by the lower drive pulley 434 to pivot themirror 402 about a horizontal axis. Similar mechanisms in the uppercounter-weight drive assembly 428 allow the upper drive pulley 424 todrive the upper tension wire back and forth to pivot the mirror 402about a tilted (off vertical) axis. The lower counter-weight driveassembly 426 includes a cylindrical drive mount 444 (FIG. 20) and aring-shaped counter-weight 446. The cylindrical drive mount 444 has ovalapertures 448 (FIG. 14) formed on opposite sides thereof to allowingress and egress of the lower tension wire 420.

The lower and upper counter-weight drive assemblies 426 and 428 arecapable of reciprocal vertical motion within the bore of the supportpost 418. A control circuit (not illustrated) receives input from a MEMSaccelerometer as previously described and causes the micro-motors of thelower and upper counter-weight drive assemblies 426 and 428 to move themirror 402 into the optimum position for reflecting solar radiation ontoa target (not illustrated in FIGS. 14-20), such as a photovoltaic array,heat exchanger, etc.

Referring now to FIG. 21, a heliostat mirror (1000) is supported by avertical tubular support (1002). A motorized tilt mechanism (not shown)provides rotation around an approximately horizontal axis (1004) fortracking solar elevation. A second motorized rotation mechanism (notshown) provides rotation around an approximately vertical axis (1006)for tracking the sun (1018) in azimuth across the sky. The mirror neednot be flat but might have some curvature in one or more directions toallow the solar energy to be brought to a tight focus. A sensor andcontrol package (1008) can be mounted anywhere on the mirror or thesupporting structure. Preferably, all the sensors and controls areintegrated into a single package to reduce cost and the need forinterconnecting cables. In this case, the sensor and control packageneeds to be mounted so that it is subject to both tilt and rotationduring mirror positioning. One or more magnetic sensors 1010 are used tosense the earth's magnetic field (1012) to determine rotational positionaround an approximately vertical axis. One or more accelerometersincluded in the sensor package 1008 are used to sense the gravity vector(1014) and determine rotational position around an approximatelyhorizontal tilt axis 1004. The magnetic sensor may be a three-axismagnetic field sensor which provides x, y, and z data in response tomeasurement of the earth's magnetic field.

A separate sensor such as the Honeywell HMC5843 available from HoneywellInternational Inc., 101 Columbia Road, Morristown, N.J. 07962 or theAK8973S available from Asahi Kasei Microsystems (AKM Semiconductor) at1731 Technology Drive, San Jose, Calif. 95110, are examples which wouldserve. Alternatively a combined integrated circuit including boththree-axis accelerometer and three-axis compass, such as the AMI6026-axis motion sensor which incorporates 3-axis magnetometer and 3-axisaccelerometer (available from Aichi Steel Corporation at 1 Wano-wari,Arao-machi, Tokai-shi, Aichi-ken, 476-8666 Japan) or theSTMicroelectronics LSM303DLH geomagnetic module combining magnetic-fieldand linear-acceleration sensing would serve. Where a combined sensorunit is used, separate magnetic sensors 1010 are dispensed with as boththe gravitic vector 1014 and the magnetic vector 1012 are measured bythe combined unit.

For maximum durability, the sensor and control package can be mountedbehind an uncoated transparent section of glass mirror. Optical sensorsto determine the position of the sun can be integrated into the sensorand control package. The same suite of optical sensors can further beused to determine the relative position of the solar energy target.Various means of building optical sensors to sense light direction areknown in the art. Optionally, photovoltaic cells can be integrated intothe sensor and control package or optionally mounted in an adjacentfashion (1011). Batteries or capacitors can be used to store the energyfrom the photovoltaic cells to provide power to operate both motorizedrotation axes. Alternatively wired power (not shown) can be used. Awireless link (1016) or a wired link (not shown) can be used to remotelycontrol each mirror and exchange data between each mirror's controlsystem and a centralized control facility. If a wireless link isemployed, a mesh networking topology is preferably used to allow dataand control signals to be communicated across a heliostat array of largearea extent. A large heliostat array might be usefully employed toproduce hydrogen fuel by photo-catalytic means.

Control signals from the mirror control system to each motorizedrotation axis might be by wireless means. For lowest possible cost andlong terms reliability it is desirable to reduce the number of cables,wires, and connectors as possible. Power to run the axis rotation motorsmay be provided by a hardwired means while control might be provided bywireless means. Rotation motors may alternatively each have their ownsmall photovoltaic panel and energy storage means.

FIG. 22 illustrates a block diagram of the control system for theembodiment of FIG. 21. In FIG. 22, a controller block (2216) receivesdata from magnetic sensors (2200) which sense rotation around anapproximately vertical axis and acceleration sensors (2202) to sensetilt around an approximately horizontal axis to provide signalsindicative of heliostat mirror position. Optionally, photo sensors(2204) may be used to establish the position of the heliostat relativeto the sun, or to a target with an optical beacon. In FIG. 22, theoption of using photovoltaics to power the system is illustratedincluding photovoltaic cells (2218), a power-conditioning block (2220),and a power storage unit (2222) which in turn communicates and suppliespower to the controller block (2216). Further, in FIG. 22, thecontroller block (2216) is illustrated with a communication link via aZigBee module (2214) which in turn is in communication with a meshnetwork (2224) allowing coordination of the local system with a remotesolar array controller (2226). The dashed line represents an alternativeenergy source (2230) by usual wired connection to the energy grid, alocal generator, or other energy supply. The controller block (2216), bymeans of emitted control signals, governs the activation of a motordrive (2208) which in turn controls two axes, one approximately vertical(2210) and one approximately horizontal (2212). An optional wirelesslink (2232) is also shown for data relay between the controller blockand the motor control electronics if separate power is available.

FIG. 23 illustrates a detailed block diagram of exemplary components ofmagnetic sensors and accelerometers and their relationship to the axismotors of an embodiment similar to FIG. 21. In FIG. 23 an integratedcircuit system 2314 contains both magnetic sensors and accelerometersand their appropriate circuitry. In FIG. 23 the integrated circuit 2314is an AMI602 6-axis sensor with integrated amplifier, 12-bitanalog-digital converter and an 8032 microprocessor controlling thecircuits of the integrated circuit 2314.

Three sensors measure values for magnetic orientation relative to theearth's magnetic field on an X axis 2318, a Y axis 2320, and a Z axis2320. Three accelerometer sensors measure acceleration at the same timealong the X axis 2324, Y axis 2326, and Z axis 2328. These sensorsignals are processed through a converter into digital data transmittedto a microcontroller 2310. In FIG. 23, the microcontroller 2310 is a64-pin PIC 18F6722 available from Microchip Technology Inc., 2355 WestChandler Blvd., Chandler, Ariz., USA 85224-6199. The microcontroller2310 communicates with the AMI602 integrated circuit 2314 through astandard I²C bus 2323. A real-time clock chip 2316 also communicateswith the microcontroller 2310 on the I²C bus 2323. In FIG. 23 thereal-time clock chip 2316 is a DS 1340C with built-in calendar andclock, available from Maxim/Dallas Semiconductor, at Maxim IntegratedProducts, Inc., 120 San Gabriel Drive, Sunnyvale, Calif. 94086.

The microcontroller 2310 also receives digital GPS time base informationfrom an optional GPS/time base receiver 2312; alternatively it mayreceive data streams from an onboard clock 2316 and perform look-upoperations using an on-board or remote data lookup table. An optionalZigBee communication module 2319 and the optional GPS/time base receiver2312 communicate to the microcontroller 2310 by means of an on-boarduniversal synchronous/asynchronous receiver/transmitter, or USART 2321.The microcontroller 2310 is capable of sending pulse-width modulateddata on two output pins, P3A and P3D, shown in FIG. 23 as a combined PWMport 2325. Motor control commands are transmitted by means of the PWMport 2325 to a first axis motor 2327 controlling the horizontal axis ofthe solar collector, and a second axis motor 2329 controlling thevertical axis thereof.

FIG. 24 is a flow chart of the control logic. By calculating the presentorientation of the sun and the measured present orientation of theline-of-direction axis of the mirror such as 1000 in FIG. 21, themicroprocessor may derive delta values for each axis to align the mirror1000 (FIG. 21) with the sun 1018 (FIG. 21). These delta values are inturn used to compute motor commands to drive the horizontal axis motor2326 and the vertical axis motor 2334 in FIG. 23. In FIG. 24 the controlcycle begins with a step 2402 of getting the real-time clock value and afollowing step 2404 of looking up the sun's real-time position andtranslating to azimuth and elevation values. A test step 2406 is done todetermine whether the sun is over the horizon or not; if it is not, themirrors are parked (step 2408) and a sleep timer is initiated (step2410). The system is placed in a sleep state 2412 until such time as asleep timer interrupt 2401 occurs. The sleep timer interrupt may becaused by passage of a specified period, or by an external signal, or apre-defined clock moment, or by a photo-sensor trigger event, forexample. If test step 2406 determines the sun is over the horizon, thesystem must then get the zero baseline value for the accelerometers(step 2414) and their current values (step 2416). Likewise, it getsbaseline values for the magnetic sensors (step 2418) on three axes, andtheir current values (step 2420). Based on these values the systemcomputes current azimuth and elevation in Step 2422, and compares themto the required azimuth and elevation in step 2424. A test is then donein Step 2426 to determine whether the delta between the required azimuthand elevation and their present values is below a pre-defined tolerancethreshold. If the delta is within tolerable limits the motors are halted(step 2428) and the sleep timer initiation step 2410 is executed. If thedelta exceeds tolerable limits the system must then compute the amountof change required in azimuth and elevation (step 2430) and translatethat change into a change for the horizontal axis and the vertical axis(step 2432). These values must in turn be translated into motor controlvalues in Step 2434 resulting in motor control transmissions sent to thefirst axis motor 2326 (FIG. 23) and the second axis motor 2328 (FIG. 23)in Step 2436.

It will be clear to one skilled in the art that the use of otherparticular sensors and other configurations will also prove workable.The particular components described are mentioned as examples ofdifferent embodiments.

While several preferred embodiments of the present invention have beendescribed, and some variations thereof, further modifications will occurto those skilled in the art. For example, any of the embodimentsdescribed herein can utilize accelerometers alone, or accelerometers andmagnetic sensors. Therefore the protection afforded the subject ininvention should only be limited in accordance with the followingclaims.

1. A solar tracking apparatus, comprising: means for collecting andreflecting incident solar radiation; means for supporting the solarradiation collecting and reflecting means for independent motion about apair of axes; accelerometer means for generating signals representativeof an amount and direction of motion of the solar radiation collectingand reflecting means about each of the axes; motor means forindependently driving the solar radiation collecting and reflectingmeans about each of the axes; tracking means for providing informationabout the current position of the Sun; and control means connected tothe accelerometer means, the motor means and the tracking means formaintaining a predetermined optimum orientation of the solar radiationcollecting and reflecting means as the Sun moves across the sky.
 2. Thesolar tracking apparatus of claim 1 wherein the solar radiationcollecting and reflecting means is configured for concentrating incidentsolar radiation.
 3. The solar tracking apparatus of claim 1 wherein theaccelerometer means is mounted on the solar radiation collecting andreflecting means.
 4. The solar tracking apparatus of claim 1 wherein thesupport means includes a pair of pivot mechanisms.
 5. The solar trackingapparatus of claim 1 wherein the accelerometer means includes a MEMSaccelerometer device
 6. The solar tracking apparatus of claim 1 whereinthe motor means includes first and second electric motors and first andsecond drive linkages for coupling the electric motors, respectively, tothe support means.
 7. The solar tracking apparatus of claim 1 whereinthe solar radiation collecting and reflecting means is supported so thatboth axes are substantially in the same horizontal plane when the solarradiation collecting and reflecting means is in a horizontalorientation.
 8. The solar tracking apparatus of claim 7 wherein thesolar radiation collecting and reflecting means is a mirror with aconfiguration selected from the group consisting of planar, parabolicand parabolic trough.
 9. The solar tracking apparatus of claim 1 andfurther comprising means for sensing a reference position of the solarradiation collecting and reflecting means relative to the earth'smagnetic field vector and for generating signals representative of thereference position and supplying them to the control means.
 10. Thesolar tracking apparatus of claim 9 wherein the accelerometer means andthe reference position sensing means are provided by three-axis sensors.11. A heliostat mirror pointing system employing both accelerometer andmagnetic sensors to determine mirror position relative to the earth'sgravitational vector and the earth's magnetic field vector.
 12. Theheliostat mirror pointing system of claim 11 wherein one axis ofrotation is approximately vertical.
 13. The heliostat mirror pointingsystem of claim 11 wherein one axis of rotation is approximatelyhorizontal.
 14. The heliostat mirror pointing system of claim 11 whereinthe magnetic sensor is used to sense rotation around said vertical axis.15. The heliostat mirror pointing system of claim 11 wherein theaccelerometer sensor is used to sense rotation around said horizontalaxis.
 16. The heliostat mirror pointing system employing 3-axis magneticsensors to determine mirror position relative to the earth's magneticfield vector.
 17. The heliostat mirror pointing system of claim 16wherein one axis of rotation is approximately vertical.
 18. Theheliostat mirror pointing system of claim 16 wherein one axis ofrotation is approximately horizontal.
 19. The heliostat mirror pointingsystem of claim 16 wherein the magnetic sensor is used to sense rotationaround horizontal and vertical axis.
 20. A heliostat mirror controlsystem employing both accelerometer and magnetic sensors to determinemirror position relative to the earth's gravitational vector and theearth's magnetic field vector.
 21. The heliostat mirror control systemof claim 20 wherein said control system is a member of a mesh network.22. The heliostat mirror control system of claim 20 wherein said controlsystem uses a wireless control means
 23. The heliostat mirror controlsystem of claim 20 wherein photovoltaic devices are used to providepower to said control system.
 24. The heliostat mirror control system ofclaim 20 wherein photovoltaic devices are used to store energy in one ormore capacitors to provide power for said control system.
 25. Theheliostat mirror control system of claim 20 wherein photovoltaic devicesare used to store energy in one or more rechargeable batteries toprovide power for said control system.
 26. A solar tracking apparatus,comprising: a reflective surface for collecting and reflecting incidentsolar radiation; a pivot mechanism that supports the reflective surfacefor independent motion about a pair of axes; an accelerometer thatgenerates signals representative of an amount and direction of motion ofthe reflective surface about each of the axes; at least one motorcoupled to independently drive the reflective surface about each of theaxes; a data source that provides information about the current positionof the Sun; and a control circuit connected to the accelerometer, themotor and the data source for maintaining a predetermined optimumorientation of the reflecting surface as the Sun moves across the sky.27. The solar tracking apparatus of claim 26 wherein the reflectivesurface is configured for concentrating incident solar radiation. 28.The solar tracking apparatus of claim 26 wherein the accelerometer ismounted on the reflective surface.
 29. The solar tracking apparatus ofclaim 26 wherein the reflective surface is supported by a pair of pivotmechanisms.
 30. The solar tracking apparatus of claim 26 wherein theaccelerometer is a MEMS accelerometer device.
 31. The solar trackingapparatus of claim 29 wherein the apparatus includes first and secondelectric motors and first and second drive linkages for coupling theelectric motors to corresponding ones of the pair of pivot mechanisms.32. The solar tracking apparatus of claim 26 wherein the reflectivesurface is supported so that both axes are substantially in the samehorizontal plane when the reflective surface is in a horizontalorientation.
 33. The solar tracking apparatus of claim 26 wherein thereflective surface is a mirror with a configuration selected from thegroup consisting of planar, parabolic and parabolic trough.
 34. Thesolar tracking apparatus of claim 26 and further comprising means forsensing a reference position of the solar radiation collecting andreflecting means relative to the earth's magnetic field vector and forgenerating signals representative of the reference position and 4supplying them to the control circuit.
 35. The solar tracking apparatusof claim 34 wherein the accelerometer and the reference position sensingmeans are provided by three-axis sensors.